Description

The gl_get_line() function is part of the libtecla(3LIB) library. If the user
is typing at a terminal, each call prompts them for an line of
input, then provides interactive editing facilities, similar to those of the UNIX
tcsh shell. In addition to simple command-line editing, it supports recall of
previously entered command lines, TAB completion of file names, and in-line wild-card
expansion of filenames. Documentation of both the user-level command-line editing features and all
user configuration options can be found on the tecla(5) manual page.

An Example

The following shows a complete example of how to use the gl_get_line()
function to get input from the user:

In the example, first the resources needed by the gl_get_line() function are
created by calling new_GetLine(). This allocates the memory used in subsequent calls
to the gl_get_line() function, including the history buffer for recording previously entered lines.
Then one or more lines are read from the user, until either
an error occurs, or the user types exit. Then finally the resources
that were allocated by new_GetLine(), are returned to the system by calling
del_GetLine(). Note the use of the NULL return value of del_GetLine() to make
glNULL. This is a safety precaution. If the program subsequently attempts
to pass gl to gl_get_line(), said function will complain, and return an error,
instead of attempting to use the deleted resource object.

The Functions Used In The Example

The new_GetLine() function creates the resources used by the gl_get_line() function and
returns an opaque pointer to the object that contains them. The maximum
length of an input line is specified by the linelen argument, and
the number of bytes to allocate for storing history lines is set by
the histlen argument. History lines are stored back-to-back in a single buffer
of this size. Note that this means that the number of history
lines that can be stored at any given time, depends on the
lengths of the individual lines. If you want to place an upper limit
on the number of lines that can be stored, see the description
of the gl_limit_history() function. If you do not want history at all,
specify histlen as zero, and no history buffer will be allocated.

On error, a message is printed to stderr and NULL is returned.

The del_GetLine() function deletes the resources that were returned by a previous
call to new_GetLine(). It always returns NULL (for example, a deleted object).
It does nothing if the gl argument is NULL.

The gl_get_line() function can be called any number of times to read
input from the user. The gl argument must have been previously returned
by a call to new_GetLine(). The prompt argument should be a normal
null-terminated string, specifying the prompt to present the user with. By default prompts
are displayed literally, but if enabled with the gl_prompt_style() function, prompts can
contain directives to do underlining, switch to and from bold fonts, or
turn highlighting on and off.

If you want to specify the initial contents of the line for
the user to edit, pass the desired string with the start_line argument.
You can then specify which character of this line the cursor is
initially positioned over by using the start_pos argument. This should be -1 if
you want the cursor to follow the last character of the start
line. If you do not want to preload the line in this
manner, send start_line as NULL, and set start_pos to -1.

The gl_get_line() function returns a pointer to the line entered by the
user, or NULL on error or at the end of the input.
The returned pointer is part of the specified gl resource object, and
thus should not be freed by the caller, or assumed to be unchanging
from one call to the next. When reading from a user at
a terminal, there will always be a newline character at the end
of the returned line. When standard input is being taken from a
pipe or a file, there will similarly be a newline unless the
input line was too long to store in the internal buffer. In the
latter case you should call gl_get_line() again to read the rest of
the line. Note that this behavior makes gl_get_line() similar to fgets(3C). When
stdin is not connected to a terminal, gl_get_line() simply calls fgets().

The Return Status Of gl_get_line()

The gl_get_line() function has two possible return values: a pointer to the
completed input line, or NULL. Additional information about what caused gl_get_line() to
return is available both by inspecting errno and by calling the gl_return_status() function.

The following are the possible enumerated values returned by gl_return_status():

GLR_NEWLINE

The last call to gl_get_line() successfully returned a completed input line.

GLR_BLOCKED

The gl_get_line() function was in non-blocking server mode, and returned early to avoid blocking the process while waiting for terminal I/O. The gl_pending_io() function can be used to see what type of I/O gl_get_line() was waiting for. See the gl_io_mode(3TECLA).

GLR_SIGNAL

A signal was caught by gl_get_line() that had an after-signal disposition of GLS_ABORT. See gl_trap_signal().

GLR_TIMEOUT

The inactivity timer expired while gl_get_line() was waiting for input, and the timeout callback function returned GLTO_ABORT. See gl_inactivity_timeout() for information about timeouts.

GLR_FDABORT

An application I/O callback returned GLFD_ABORT. Ssee gl_watch_fd().

GLR_EOF

End of file reached. This can happen when input is coming from a file or a pipe, instead of the terminal. It also occurs if the user invokes the list-or-eof or del-char-or-list-or-eof actions at the start of a new line.

When gl_return_status() returns GLR_ERROR and the value of errno is not sufficient
to explain what happened, you can use the gl_error_message() function to request
a description of the last error that occurred.

The return value of gl_error_message() is a pointer to the message that
occurred. If the buff argument is NULL, this will be a pointer
to a buffer within gl whose value will probably change on the next
call to any function associated with gl_get_line(). Otherwise, if a non-null buff
argument is provided, the error message, including a '\0' terminator, will be
written within the first n elements of this buffer, and the return value
will be a pointer to the first element of this buffer. If
the message will not fit in the provided buffer, it will be
truncated to fit.

Optional Prompt Formatting

Whereas by default the prompt string that you specify is displayed literally
without any special interpretation of the characters within it, the gl_prompt_style() function
can be used to enable optional formatting directives within the prompt.

The style argument, which specifies the formatting style, can take any of
the following values:

GL_FORMAT_PROMPT

In this style, the formatting directives described below, when included in prompt strings, are interpreted as follows:

%B

Display subsequent characters with a bold font.

%b

Stop displaying characters with the bold font.

%F

Make subsequent characters flash.

%f

Turn off flashing characters.

%U

Underline subsequent characters.

%u

Stop underlining characters.

%P

Switch to a pale (half brightness) font.

%p

Stop using the pale font.

%S

Highlight subsequent characters (also known as standout mode).

%s

Stop highlighting characters.

%V

Turn on reverse video.

%v

Turn off reverse video.

%%

Display a single % character.

For example, in this mode, a prompt string like “%UOK%u$” would display the prompt “OK$”, but with the OK part underlined.

Note that although a pair of characters that starts with a % character, but does not match any of the above directives is displayed literally, if a new directive is subsequently introduced which does match, the displayed prompt will change, so it is better to always use %% to display a literal %.

Also note that not all terminals support all of these text attributes, and that some substitute a different attribute for missing ones.

GL_LITERAL_PROMPT

In this style, the prompt string is printed literally. This is the default style.

Alternate Configuration Sources

By default users have the option of configuring the behavior of gl_get_line()
with a configuration file called .teclarc in their home directories. The fact
that all applications share this same configuration file is both an advantage
and a disadvantage. In most cases it is an advantage, since it encourages
uniformity, and frees the user from having to configure each application separately.
In some applications, however, this single means of configuration is a problem.
This is particularly true of embedded software, where there's no filesystem to read
a configuration file from, and also in applications where a radically different
choice of keybindings is needed to emulate a legacy keyboard interface. To
cater for such cases, the gl_configure_getline() function allows the application to control
where configuration information is read from.

The gl_configure_getline() function allows the configuration commands that would normally be read
from a user's ~/.teclarc file, to be read from any or none
of, a string, an application specific configuration file, and/or a user-specific configuration file.
If this function is called before the first call to gl_get_line(), the
default behavior of reading ~/.teclarc on the first call to gl_get_line() is
disabled, so all configurations must be achieved using the configuration sources specified with
this function.

If app_string != NULL, then it is interpreted as a string containing
one or more configuration commands, separated from each other in the string
by embedded newline characters. If app_file != NULL then it is
interpreted as the full pathname of an application-specific configuration file. If user_file
!= NULL then it is interpreted as the full path name of
a user-specific configuration file, such as ~/.teclarc. For example, in the call

The app_string argument causes the calling application to start in vi(1) edit-mode,
instead of the default emacs mode, and turns off the use of the
terminal bell by the library. It then attempts to read system-wide configuration
commands from an optional file called /usr/share/myapp/teclarc, then finally reads user-specific configuration
commands from an optional .teclarc file in the user's home directory. Note that
the arguments are listed in ascending order of priority, with the contents
of app_string being potentially over riden by commands in app_file, and commands
in app_file potentially being overriden by commands in user_file.

You can call this function as many times as needed, the results
being cumulative, but note that copies of any file names specified with
the app_file and user_file arguments are recorded internally for subsequent use by
the read-init-files key-binding function, so if you plan to call this function multiple
times, be sure that the last call specifies the filenames that you
want re-read when the user requests that the configuration files be re-read.

Individual key sequences can also be bound and unbound using the gl_bind_keyseq()
function. The origin argument specifies the priority of the binding, according to
whom it is being established for, and must be one of the
following two values.

GL_USER_KEY

The user requested this key-binding.

GL_APP_KEY

This is a default binding set by the application.

When both user and application bindings for a given key sequence have
been specified, the user binding takes precedence. The application's binding is subsequently
reinstated if the user's binding is later unbound with either another call
to this function, or a call to gl_configure_getline().

The keyseq argument specifies the key sequence to be bound or unbound,
and is expressed in the same way as in a ~/.teclarc configuration
file. The action argument must either be a string containing the name
of the action to bind the key sequence to, or it must be
NULL or "" to unbind the key sequence.

Customized Word Completion

If in your application you would like to have TAB completion complete
other things in addition to or instead of filenames, you can arrange
this by registering an alternate completion callback function with a call to
the gl_customize_completion() function.

The data argument provides a way for your application to pass arbitrary,
application-specific information to the callback function. This is passed to the callback
every time that it is called. It might for example point to
the symbol table from which possible completions are to be sought. The match_fn
argument specifies the callback function to be called. The CplMatchFn function type
is defined in <libtecla.h>, as is a CPL_MATCH_FN() macro that you can
use to declare and prototype callback functions. The declaration and responsibilities of callback
functions are described in depth on the cpl_complete_word(3TECLA) manual page.

The callback function is responsible for looking backwards in the input line
from the point at which the user pressed TAB, to find the
start of the word being completed. It then must lookup possible completions
of this word, and record them one by one in the WordCompletion
object that is passed to it as an argument, by calling the cpl_add_completion()
function. If the callback function wants to provide filename completion in addition
to its own specific completions, it has the option of itself calling
the builtin filename completion callback. This also is documented on the cpl_complete_word(3TECLA) manual
page.

If you would like gl_get_line() to return the current input line when
a successful completion is been made, you can arrange this when you
call cpl_add_completion() by making the last character of the continuation suffix a
newline character. The input line will be updated to display the completion, together
with any contiuation suffix up to the newline character, and gl_get_line() will
return this input line.

If your callback function needs to write something to the terminal, it
must call gl_normal_io() before doing so. This will start a new line
after the input line that is currently being edited, reinstate normal terminal
I/O, and notify gl_get_line() that the input line will need to be redrawn
when the callback returns.

Adding Completion Actions

In the previous section the ability to customize the behavior of the
only default completion action, complete-word, was described. In this section the ability
to install additional action functions, so that different types of word completion
can be bound to different key sequences, is described. This is achieved by
using the gl_completion_action() function.

The data and match_fn arguments are as described on the cpl_complete_word(3TECLA) manual
page, and specify the callback function that should be invoked to identify possible
completions. The list_only argument determines whether the action that is being defined
should attempt to complete the word as far as possible in the
input line before displaying any possible ambiguous completions, or whether it should simply
display the list of possible completions without touching the input line. The
former option is selected by specifying a value of 0, and the
latter by specifying a value of 1. The name argument specifies the
name by which configuration files and future invocations of this function should refer
to the action. This must either be the name of an existing
completion action to be changed, or be a new unused name for
a new action. Finally, the keyseq argument specifies the default key sequence
to bind the action to. If this is NULL, no new key sequence
will be bound to the action.

Beware that in order for the user to be able to change
the key sequence that is bound to actions that are installed in
this manner, you shouldcall gl_completion_action() to install a given action for the
first time between calling new_GetLine() and the first call to gl_get_line(). Otherwise, when
the user's configuration file is read on the first call to gl_get_line(),
the name of the your additional action will not be known, and
any reference to it in the configuration file will generate an error.

As discussed for gl_customize_completion(), if your callback function needs to write anything
to the terminal, it must call gl_normal_io() before doing so.

Defining Custom Actions

Although the built-in key-binding actions are sufficient for the needs of most
applications, occasionally a specialized application may need to define one or more
custom actions, bound to application-specific key sequences. For example, a sales application
would benefit from having a key sequence that displayed the part name that
corresponded to a part number preceding the cursor. Such a feature is
clearly beyond the scope of the built-in action functions. So for such
special cases, the gl_register_action() function is provided.

The gl_register_action() function lets the application register an external function, fn, that
will thereafter be called whenever either the specified key sequence, keyseq, is
entered by the user, or the user enters any other key sequence that
the user subsequently binds to the specified action name, name, in their
configuration file. The data argument can be a pointer to anything that
the application wants to have passed to the action function, fn, whenever that
function is invoked.

The action function, fn, should be declared using the GL_ACTION_FN() macro, which
is defined in <libtecla.h>.

The gl and data arguments are those that were previously passed to
gl_register_action() when the action function was registered. The count argument is a
numeric argument which the user has the option of entering using the digit-argument
action, before invoking the action. If the user does not enter a
number, then the count argument is set to 1. Nominally this argument
is interpreted as a repeat count, meaning that the action should be
repeated that many times. In practice however, for some actions a repeat count
makes little sense. In such cases, actions can either simply ignore the
count argument, or use its value for a different purpose.

A copy of the current input line is passed in the read-only
line argument. The current cursor position within this string is given by
the index contained in the curpos argument. Note that direct manipulation of
the input line and the cursor position is not permitted because the rules
dictated by various modes (such as vi mode versus emacs mode, no-echo
mode, and insert mode versus overstrike mode) make it too complex for
an application writer to write a conforming editing action, as well as
constrain future changes to the internals of gl_get_line(). A potential solution to this
dilemma would be to allow the action function to edit the line
using the existing editing actions. This is currently under consideration.

If the action function wishes to write text to the terminal without
this getting mixed up with the displayed text of the input line,
or read from the terminal without having to handle raw terminal I/O,
then before doing either of these operations, it must temporarily suspend line
editing by calling the gl_normal_io() function. This function flushes any pending output to
the terminal, moves the cursor to the start of the line that
follows the last terminal line of the input line, then restores the
terminal to a state that is suitable for use with the C
stdio facilities. The latter includes such things as restoring the normal mapping
of \n to \r\n, and, when in server mode, restoring the normal
blocking form of terminal I/O. Having called this function, the action function
can read from and write to the terminal without the fear of creating
a mess. It is not necessary for the action function to restore
the original editing environment before it returns. This is done automatically by
gl_get_line() after the action function returns. The following is a simple example
of an action function which writes the sentence “Hello world” on a new
terminal line after the line being edited. When this function returns, the
input line is redrawn on the line that follows the “Hello world”
line, and line editing resumes.

Action functions must return one of the following values, to tell gl_get_line()
how to proceed.

GLA_ABORT

Cause gl_get_line() to return NULL.

GLA_RETURN

Cause gl_get_line() to return the completed input line

GLA_CONTINUE

Resume command-line editing.

Note that the name argument of gl_register_action() specifies the name by which
a user can refer to the action in their configuration file. This
allows them to re-bind the action to an alternate key-seqeunce. In order
for this to work, it is necessary to call gl_register_action() between calling new_GetLine()
and the first call to gl_get_line().

History Files

To save the contents of the history buffer before quitting your application
and subsequently restore them when you next start the application, the gl_save_history()
and gl_load_history() functions are provided.

The filename argument specifies the name to give the history file when
saving, or the name of an existing history file, when loading. This
may contain home directory and environment variable expressions, such as ~/.myapp_history or
$HOME/.myapp_history.

Along with each history line, additional information about it, such as its
nesting level and when it was entered by the user, is recorded
as a comment preceding the line in the history file. Writing this
as a comment allows the history file to double as a command
file, just in case you wish to replay a whole session using it.
Since comment prefixes differ in different languages, the comment argument is provided
for specifying the comment prefix. For example, if your application were a
UNIX shell, such as the Bourne shell, you would specify “#”
here. Whatever you choose for the comment character, you must specify the same
prefix to gl_load_history() that you used when you called gl_save_history() to write
the history file.

The max_lines argument must be either -1 to specify that all lines
in the history list be saved, or a positive number specifying a
ceiling on how many of the most recent lines should be saved.

Both fuctions return non-zero on error, after writing an error message to
stderr. Note that gl_load_history() does not consider the non-existence of a file
to be an error.

Multiple History Lists

If your application uses a single GetLine object for entering many different
types of input lines, you might want gl_get_line() to distinguish the different
types of lines in the history list, and only recall lines that
match the current type of line. To support this requirement, gl_get_line() marks lines
being recorded in the history list with an integer identifier chosen by
the application. Initially this identifier is set to 0 by new_GetLine(), but
it can be changed subsequently by calling gl_group_history().

The integer identifier ID can be any number chosen by the application,
but note that gl_save_history() and gl_load_history() preserve the association between identifiers and
historical input lines between program invocations, so you should choose fixed identifiers for
the different types of input line used by your application.

Whenever gl_get_line() appends a new input line to the history list, the
current history identifier is recorded with it, and when it is asked
to recall a historical input line, it only recalls lines that are
marked with the current identifier.

Displaying History

The history list can be displayed by calling gl_show_history(). This function displays
the current contents of the history list to the stdio output stream
fp. If the max_lines argument is greater than or equal to zero, then
no more than this number of the most recent lines will
be displayed. If the all_groups argument is non-zero, lines from all history
groups are displayed. Otherwise only those of the currently selected history group
are displayed. The format string argument, fmt, determines how the line is
displayed. This can contain arbitrary characters which are written verbatim, interleaved with
any of the following format directives:

%D

The date on which the line was originally entered, formatted like 2001-11-20.

%T

The time of day when the line was entered, formatted like 23:59:59.

%N

The sequential entry number of the line in the history buffer.

%G

The number of the history group which the line belongs to.

%%

A literal % character.

%H

The history line itself.

Thus a format string like “%D %T %H0” would output something like:

2001-11-20 10:23:34 Hello world

Note the inclusion of an explicit newline character in the format string.

Looking Up History

The gl_lookup_history() function allows the calling application to look up lines in
the history list.

The id argument indicates which line to look up, where the first
line that was entered in the history list after new_GetLine() was called
is denoted by 0, and subsequently entered lines are denoted with successively
higher numbers. Note that the range of lines currently preserved in the history
list can be queried by calling the gl_range_of_history() function. If the requested
line is in the history list, the details of the line are
recorded in the variable pointed to by the hline argument, and 1 is
returned. Otherwise 0 is returned, and the variable pointed to by hline
is left unchanged.

Beware that the string returned in hline->line is part of the history
buffer, so it must not be modified by the caller, and will
be recycled on the next call to any function that takes gl
as its argument. Therefore you should make a private copy of this string
if you need to keep it.

Manual History Archival

By default, whenever a line is entered by the user, it is
automatically appended to the history list, just before gl_get_line() returns the line
to the caller. This is convenient for the majority of applications, but
there are also applications that need finer-grained control over what gets added to
the history list. In such cases, the automatic addition of entered lines
to the history list can be turned off by calling the gl_automatic_history()
function.

If this function is called with its enable argument set to 0,
gl_get_line() will not automatically archive subsequently entered lines. Automatic archiving can be
reenabled at a later time by calling this function again, with its
enable argument set to 1. While automatic history archiving is disabled, the calling
application can use the gl_append_history() to append lines to the history list
as needed.

The line argument specifies the line to be added to the history
list. This must be a normal '\0 ' terminated string. If this
string contains any newline characters, the line that gets archived in the
history list will be terminated by the first of these. Otherwise it will
be terminated by the '\0 ' terminator. If the line is longer
than the maximum input line length that was specified when new_GetLine() was
called, it will be truncated to the actual gl_get_line() line length when
the line is recalled.

If successful, gl_append_history() returns 0. Otherwise it returns non-zero and sets errno
to one of the following values.

EINVAL

One of the arguments passed to gl_append_history() was NULL.

ENOMEM

The specified line was longer than the allocated size of the history buffer (as specified when new_GetLine() was called), so it could not be archived.

A textual description of the error can optionally be obtained by calling
gl_error_message(). Note that after such an error, the history list remains in
a valid state to receive new history lines, so there is little
harm in simply ignoring the return status of gl_append_history().

Miscellaneous History Configuration

If you wish to change the size of the history buffer that
was originally specified in the call to new_GetLine(), you can do so
with the gl_resize_history() function.

The histlen argument specifies the new size in bytes, and if you
specify this as 0, the buffer will be deleted.

As mentioned in the discussion of new_GetLine(), the number of lines that
can be stored in the history buffer, depends on the lengths of
the individual lines. For example, a 1000 byte buffer could equally store
10 lines of average length 100 bytes, or 20 lines of average length
50 bytes. Although the buffer is never expanded when new lines are
added, a list of pointers into the buffer does get expanded when
needed to accomodate the number of lines currently stored in the buffer.
To place an upper limit on the number of lines in the
buffer, and thus a ceiling on the amount of memory used in this
list, you can call the gl_limit_history() function.

The max_lines should either be a positive number >= 0, specifying an
upper limit on the number of lines in the buffer, or be
-1 to cancel any previously specified limit. When a limit is in
effect, only the max_lines most recently appended lines are kept in the buffer.
Older lines are discarded.

To discard lines from the history buffer, use the gl_clear_history() function.

The all_groups argument tells the function whether to delete just the lines
associated with the current history group (see gl_group_history()) or all historical lines
in the buffer.

The gl_toggle_history() function allows you to toggle history on and off without
losing the current contents of the history list.

Setting the enable argument to 0 turns off the history mechanism, and
setting it to 1 turns it back on. When history is turned
off, no new lines will be added to the history list, and
history lookup key-bindings will act as though there is nothing in the
history buffer.

Querying History Information

The configured state of the history list can be queried with the
gl_history_state() function. On return, the status information is recorded in the variable
pointed to by the state argument.

The gl_range_of_history() function returns the number and range of lines in the
history list. The return values are recorded in the variable pointed to
by the range argument. If the nlines member of this structure is
greater than zero, then the oldest and newest members report the range of
lines in the list, and newest=oldest+nlines-1. Otherwise they are both zero.

The gl_size_of_history() function returns the total size of the history buffer and
the amount of the buffer that is currently occupied.

On return, the size information is recorded in the variable pointed to
by the size argument.

Changing Terminals

The new_GetLine() constructor function assumes that input is to be read from
stdin and output written to stdout. The following function allows you to
switch to different input and output streams.

The gl argument is the object that was returned by new_GetLine(). The
input_fp argument specifies the stream to read from, and output_fp specifies the
stream to be written to. Only if both of these refer to a
terminal, will interactive terminal input be enabled. Otherwise gl_get_line() will simply call
fgets() to read command input. If both streams refer to a terminal,
then they must refer to the same terminal, and the type of
this terminal must be specified with the term argument. The value of the
term argument is looked up in the terminal information database (terminfo or
termcap), in order to determine which special control sequences are needed to
control various aspects of the terminal. new_GetLine() for example, passes the return value
of getenv(“TERM”) in this argument. Note that if one or both of
input_fp and output_fp do not refer to a terminal, then it is legal
to pass NULL instead of a terminal type.

Note that if you want to pass file descriptors to gl_change_terminal(), you
can do this by creating stdio stream wrappers using the POSIX fdopen(3C)
function.

External Event Handling

By default, gl_get_line() does not return until either a complete input line
has been entered by the user, or an error occurs. In programs
that need to watch for I/O from other sources than the terminal,
there are two options.

Use the functions described in the gl_io_mode(3TECLA) manual page to switch gl_get_line() into non-blocking server mode. In this mode, gl_get_line() becomes a non-blocking, incremental line-editing function that can safely be called from an external event loop. Although this is a very versatile method, it involves taking on some responsibilities that are normally performed behind the scenes by gl_get_line().

While gl_get_line() is waiting for keyboard input from the user, you can ask it to also watch for activity on arbitrary file descriptors, such as network sockets or pipes, and have it call functions of your choosing when activity is seen. This works on any system that has the select system call, which is most, if not all flavors of UNIX.

Registering a file descriptor to be watched by gl_get_line() involves calling the
gl_watch_fd() function. If this returns non-zero, then it means that either your
arguments are invalid, or that this facility is not supported on the
host system.

The fd argument is the file descriptor to be watched. The event
argument specifies what type of activity is of interest, chosen from the
following enumerated values:

GLFD_READ

Watch for the arrival of data to be read.

GLFD_WRITE

Watch for the ability to write to the file descriptor without blocking.

GLFD_URGENT

Watch for the arrival of urgent out-of-band data on the file descriptor.

The callback argument is the function to call when the selected activity
is seen. It should be defined with the following macro, which is
defined in libtecla.h.

The data argument of the gl_watch_fd() function is passed to the callback
function for its own use, and can point to anything you like,
including NULL. The file descriptor and the event argument are also passed
to the callback function, and this potentially allows the same callback function to
be registered to more than one type of event and/or more than
one file descriptor. The return value of the callback function should be
one of the following values.

GLFD_ABORT

Tell gl_get_line() to abort. When this happens, gl_get_line() returns NULL, and a following call to gl_return_status() will return GLR_FDABORT. Note that if the application needs errno always to have a meaningful value when gl_get_line() returns NULL, the callback function should set errno appropriately.

GLFD_REFRESH

Redraw the input line then continue waiting for input. Return this if your callback wrote to the terminal.

GLFD_CONTINUE

Continue to wait for input, without redrawing the line.

Note that before calling the callback, gl_get_line() blocks most signals and leaves
its own signal handlers installed, so if you need to catch a
particular signal you will need to both temporarily install your own signal
handler, and unblock the signal. Be sure to re-block the signal (if it
was originally blocked) and reinstate the original signal handler, if any, before
returning.

Your callback should not try to read from the terminal, which is
left in raw mode as far as input is concerned. You can
write to the terminal as usual, since features like conversion of newline
to carriage-return/linefeed are re-enabled while the callback is running. If your callback
function does write to the terminal, be sure to output a newline first,
and when your callback returns, tell gl_get_line() that the input line needs
to be redrawn, by returning the GLFD_REFRESH status code.

To remove a callback function that you previously registered for a given
file descriptor and event, simply call gl_watch_fd() with the same fd and
event arguments, but with a callback argument of 0. The data argument
is ignored in this case.

Setting An Inactivity Timeout

The gl_inactivity_timeout() function can be used to set or cancel an inactivity
timeout. Inactivity in this case refers both to keyboard input, and to
I/O on any file descriptors registered by prior and subsequent calls to
gl_watch_fd().

The timeout is specified in the form of an integral number of
seconds and an integral number of nanoseconds, specified by the sec and
nsec arguments, respectively. Subsequently, whenever no activity is seen for this time
period, the function specified by the callback argument is called. The data argument
of gl_inactivity_timeout() is passed to this callback function whenever it is invoked,
and can thus be used to pass arbitrary application-specific information to the callback.
The following macro is provided in <libtecla.h> for applications to use to
declare and prototype timeout callback functions.

On returning, the application's callback is expected to return one of the
following enumerators to tell gl_get_line() how to procede after the timeout has
been handled by the callback.

GLTO_ABORT

Tell gl_get_line() to abort. When this happens, gl_get_line() will return NULL, and a following call to gl_return_status() will return GLR_TIMEOUT. Note that if the application needs errno always to have a meaningful value when gl_get_line() returns NULL, the callback function should set errno appropriately.

GLTO_REFRESH

Redraw the input line, then continue waiting for input. You should return this value if your callback wrote to the terminal.

GLTO_CONTINUE

In normal blocking-I/O mode, continue to wait for input, without redrawing the user's input line. In non-blocking server I/O mode (see gl_io_mode(3TECLA)), gl_get_line() acts as though I/O blocked. This means that gl_get_line() will immediately return NULL, and a following call to gl_return_status() will return GLR_BLOCKED.

Note that before calling the callback, gl_get_line() blocks most signals and leaves
its own signal handlers installed, so if you need to catch a
particular signal you will need to both temporarily install your own signal
handler and unblock the signal. Be sure to re-block the signal (if it
was originally blocked) and reinstate the original signal handler, if any, before
returning.

Your callback should not try to read from the terminal, which is
left in raw mode as far as input is concerned. You can
however write to the terminal as usual, since features like conversion of
newline to carriage-return/linefeed are re-enabled while the callback is running. If your
callback function does write to the terminal, be sure to output a newline
first, and when your callback returns, tell gl_get_line() that the input line
needs to be redrawn, by returning the GLTO_REFRESH status code.

Finally, note that although the timeout arguments include a nanosecond component, few
computer clocks presently have resolutions that are finer than a few milliseconds,
so asking for less than a few milliseconds is equivalent to requesting
zero seconds on many systems. If this would be a problem, you should
base your timeout selection on the actual resolution of the host clock
(for example, by calling sysconf(_SC_CLK_TCK)).

To turn off timeouts, simply call gl_inactivity_timeout() with a callback argument of
0. The data argument is ignored in this case.

Signal Handling Defaults

By default, the gl_get_line() function intercepts a number of signals. This is
particularly important for signals that would by default terminate the process, since
the terminal needs to be restored to a usable state before this
happens. This section describes the signals that are trapped by default and how
gl_get_line() responds to them. Changing these defaults is the topic of the
following section.

When the following subset of signals are caught, gl_get_line() first restores the
terminal settings and signal handling to how they were before gl_get_line() was
called, resends the signal to allow the calling application's signal handlers to
handle it, then, if the process still exists, returns NULL and sets errno
as specified below.

SIGINT

This signal is generated both by the keyboard interrupt key (usually ^C), and the keyboard break key. The errno value is EINTR.

SIGHUP

This signal is generated when the controlling terminal exits. The errno value is ENOTTY.

SIGPIPE

This signal is generated when a program attempts to write to a pipe whose remote end is not being read by any process. This can happen for example if you have called gl_change_terminal() to redirect output to a pipe hidden under a pseudo terminal. The errno value is EPIPE.

SIGQUIT

This signal is generated by the keyboard quit key (usually ^\\). The errno value is EINTR.

SIGABRT

This signal is generated by the standard C, abort function. By default it both terminates the process and generates a core dump. The errno value is EINTR.

SIGTERM

This is the default signal that the UNIX kill command sends to processes. The errno value is EINTR.

Note that in the case of all of the above signals, POSIX
mandates that by default the process is terminated, with the addition of
a core dump in the case of the SIGQUIT signal. In other
words, if the calling application does not override the default handler by
supplying its own signal handler, receipt of the corresponding signal will terminate the
application before gl_get_line() returns.

If gl_get_line() aborts with errno set to EINTR, you can find out
what signal caused it to abort, by calling the gl_last_signal() function. This
returns the numeric code (for example, SIGINT) of the last signal that was
received during the most recent call to gl_get_line(), or -1 if no
signals were received.

On systems that support it, when a SIGWINCH (window change) signal is
received, gl_get_line() queries the terminal to find out its new size, redraws
the current input line to accomodate the new size, then returns to
waiting for keyboard input from the user. Unlike other signals, this signal is
not resent to the application.

Finally, the following signals cause gl_get_line() to first restore the terminal and
signal environment to that which prevailed before gl_get_line() was called, then resend
the signal to the application. If the process still exists after the
signal has been delivered, then gl_get_line() then re-establishes its own signal handlers, switches
the terminal back to raw mode, redisplays the input line, and goes
back to awaiting terminal input from the user.

SIGCONT

This signal is generated when a suspended process is resumed.

SIGPOLL

On SVR4 systems, this signal notifies the process of an asynchronous I/O event. Note that under 4.3+BSD, SIGIO and SIGPOLL are the same. On other systems, SIGIO is ignored by default, so gl_get_line() does not trap it by default.

SIGPWR

This signal is generated when a power failure occurs (presumably when the system is on a UPS).

SIGALRM

This signal is generated when a timer expires.

SIGUSR1

An application specific signal.

SIGUSR2

Another application specific signal.

SIGVTALRM

This signal is generated when a virtual timer expires. See setitimer(2).

SIGXCPU

This signal is generated when a process exceeds its soft CPU time limit.

SIGXFSZ

This signal is generated when a process exceeds its soft file-size limit.

SIGTSTP

This signal is generated by the terminal suspend key, which is usually ^Z, or the delayed terminal suspend key, which is usually ^Y.

SIGTTIN

This signal is generated if the program attempts to read from the terminal while the program is running in the background.

SIGTTOU

This signal is generated if the program attempts to write to the terminal while the program is running in the background.

Obviously not all of the above signals are supported on all systems,
so code to support them is conditionally compiled into the tecla library.

Note that if SIGKILL or SIGPOLL, which by definition cannot be caught,
or any of the hardware generated exception signals, such as SIGSEGV, SIGBUS,
and SIGFPE, are received and unhandled while gl_get_line() has the terminal in
raw mode, the program will be terminated without the terminal having been
restored to a usable state. In practice, job-control shells usually reset the
terminal settings when a process relinquishes the controlling terminal, so this is
only a problem with older shells.

Customized Signal Handling

The previous section listed the signals that gl_get_line() traps by default, and
described how it responds to them. This section describes how to both
add and remove signals from the list of trapped signals, and how
to specify how gl_get_line() should respond to a given signal.

If you do not need gl_get_line() to do anything in response to
a signal that it normally traps, you can tell to gl_get_line() to
ignore that signal by calling gl_ignore_signal().

The signo argument is the number of the signal (for example, SIGINT)
that you want to have ignored. If the specified signal is not
currently one of those being trapped, this function does nothing.

The gl_trap_signal() function allows you to either add a new signal to
the list that gl_get_line() traps or modify how it responds to a
signal that it already traps.

The signo argument is the number of the signal that you want
to have trapped. The flags argument is a set of flags that
determine the environment in which the application's signal handler is invoked. The
after argument tells gl_get_line() what to do after the application's signal handler returns.
The errno_value tells gl_get_line() what to set errno to if told to
abort.

The flags argument is a bitwise OR of zero or more of
the following enumerators:

GLS_RESTORE_SIG

Restore the caller's signal environment while handling the signal.

GLS_RESTORE_TTY

Restore the caller's terminal settings while handling the signal.

GLS_RESTORE_LINE

Move the cursor to the start of the line following the input line before invoking the application's signal handler.

GLS_REDRAW_LINE

Redraw the input line when the application's signal handler returns.

GLS_UNBLOCK_SIG

Normally, if the calling program has a signal blocked (see sigprocmask(2)), gl_get_line() does not trap that signal. This flag tells gl_get_line() to trap the signal and unblock it for the duration of the call to gl_get_line().

GLS_DONT_FORWARD

If this flag is included, the signal will not be forwarded to the signal handler of the calling program.

Two commonly useful flag combinations are also enumerated as follows:

GLS_RESTORE_ENV

GLS_RESTORE_SIG | GLS_RESTORE_TTY |GLS_REDRAW_LINE

GLS_SUSPEND_INPUT

GLS_RESTORE_ENV | GLS_RESTORE_LINE

If your signal handler, or the default system signal handler for this
signal, if you have not overridden it, never either writes to the
terminal, nor suspends or terminates the calling program, then you can safely
set the flags argument to 0.

The cursor does not get left in the middle of the input line.

So that the user can type in input and have it echoed.

So that you do not need to end each output line with \r\n, instead of just \n.

The GL_RESTORE_ENV combination is the same as GL_SUSPEND_INPUT, except that it does
not move the cursor. If your signal handler does not read or
write anything to the terminal, the user will not see any visible
indication that a signal was caught. This can be useful if you have
a signal handler that only occasionally writes to the terminal, where using
GL_SUSPEND_LINE would cause the input line to be unnecessarily duplicated when nothing
had been written to the terminal. Such a signal handler, when it
does write to the terminal, should be sure to start a new line
at the start of its first write, by writing a new line
before returning. If the signal arrives while the user is entering a
line that only occupies a signal terminal line, or if the cursor
is on the last terminal line of a longer input line, this
will have the same effect as GL_SUSPEND_INPUT. Otherwise it will start writing on
a line that already contains part of the displayed input line. This
does not do any harm, but it looks a bit ugly, which
is why the GL_SUSPEND_INPUT combination is better if you know that you are
always going to be writting to the terminal.

The after argument, which determines what gl_get_line() does after the application's signal
handler returns (if it returns), can take any one of the
following values:

GLS_RETURN

Return the completed input line, just as though the user had pressed the return key.

GLS_ABORT

Cause gl_get_line() to abort. When this happens, gl_get_line() returns NULL, and a following call to gl_return_status() will return GLR_SIGNAL. Note that if the application needs errno always to have a meaningful value when gl_get_line() returns NULL, the callback function should set errno appropriately.

GLS_CONTINUE

Resume command line editing.

The errno_value argument is intended to be combined with the GLS_ABORT option,
telling gl_get_line() what to set the standard errno variable to before returning
NULL to the calling program. It can also, however, be used with the
GL_RETURN option, in case you want to have a way to distinguish
between an input line that was entered using the return key, and
one that was entered by the receipt of a signal.

Reliable Signal Handling

Signal handling is suprisingly hard to do reliably without race conditions. In
gl_get_line() a lot of care has been taken to allow applications to
perform reliable signal handling around gl_get_line(). This section explains how to make
use of this.

As an example of the problems that can arise if the application
is not written correctly, imagine that one's application has a SIGINT signal
handler that sets a global flag. Now suppose that the application tests
this flag just before invoking gl_get_line(). If a SIGINT signal happens to be
received in the small window of time between the statement that tests
the value of this flag, and the statement that calls gl_get_line(), then
gl_get_line() will not see the signal, and will not be interrupted. As a
result, the application will not be able to respond to the signal
until the user gets around to finishing entering the input line and
gl_get_line() returns. Depending on the application, this might or might not be
a disaster, but at the very least it would puzzle the user.

The way to avoid such problems is to do the following.

If needed, use the gl_trap_signal() function to configure gl_get_line() to abort when important signals are caught.

Configure gl_get_line() such that if any of the signals that it catches are blocked when gl_get_line() is called, they will be unblocked automatically during times when gl_get_line() is waiting for I/O. This can be done either on a per signal basis, by calling the gl_trap_signal() function, and specifying the GLS_UNBLOCK attribute of the signal, or globally by calling the gl_catch_blocked() function. This function simply adds the GLS_UNBLOCK attribute to all of the signals that it is currently configured to trap.

Just before calling gl_get_line(), block delivery of all of the signals that gl_get_line() is configured to trap. This can be done using the POSIX sigprocmask function in conjunction with the gl_list_signals() function. This function returns the set of signals that it is currently configured to catch in the set argument, which is in the form required by sigprocmask(2).

In the example, one would now test the global flag that the signal handler sets, knowing that there is now no danger of this flag being set again until gl_get_line() unblocks its signals while performing I/O.

Eventually gl_get_line() returns, either because a signal was caught, an error occurred, or the user finished entering their input line.

Now one would check the global signal flag again, and if it is set, respond to it, and zero the flag.

Use sigprocmask() to unblock the signals that were blocked in step 3.

The same technique can be used around certain POSIX signal-aware functions, such
as sigsetjmp(3C) and sigsuspend(2), and in particular, the former of these two
functions can be used in conjunction with siglongjmp(3C) to implement race-condition free
signal handling around other long-running system calls. The gl_get_line() function manages to
reliably trap signals around calls to functions like read(2) and select(3C) without
race conditions.

The gl_get_line() function first uses the POSIX sigprocmask() function to block the
delivery of all of the signals that it is currently configured to
catch. This is redundant if the application has already blocked them, but
it does no harm. It undoes this step just before returning.

Whenever gl_get_line() needs to call read or select to wait for input
from the user, it first calls the POSIX sigsetjmp() function, being sure
to specify a non-zero value for its savemask argument.

If sigsetjmp() returns zero, gl_get_line() then does the following.

It uses the POSIX sigaction(2) function to register a temporary signal handler to all of the signals that it is configured to catch. This signal handler does two things.

It records the number of the signal that was received in a file-scope variable.

It then calls the POSIX siglongjmp() function using the buffer that was passed to sigsetjmp() for its first argument and a non-zero value for its second argument.

When this signal handler is registered, the sa_mask member of the struct sigactionact argument of the call to sigaction() is configured to contain all of the signals that gl_get_line() is catching. This ensures that only one signal will be caught at once by our signal handler, which in turn ensures that multiple instances of our signal handler do not tread on each other's toes.

Now that the signal handler has been set up, gl_get_line() unblocks all of the signals that it is configured to catch.

It then calls the read() or select() function to wait for keyboard input.

If this function returns (that is, no signal is received), gl_get_line() blocks delivery of the signals of interest again.

It then reinstates the signal handlers that were displaced by the one that was just installed.

Alternatively, if sigsetjmp() returns non-zero, this means that one of the signals
being trapped was caught while the above steps were executing. When this
happens, gl_get_line() does the following.

First, note that when a call to siglongjmp() causes sigsetjmp() to return,
provided that the savemask argument of sigsetjmp() was non-zero, the signal process
mask is restored to how it was when sigsetjmp() was called. This is
the important difference between sigsetjmp() and the older problematic setjmp(3C), and is
the essential ingredient that makes it possible to avoid signal handling race conditions.
Because of this we are guaranteed that all of the signals that
we blocked before calling sigsetjmp() are blocked again as soon as any
signal is caught. The following statements, which are then executed, are thus
guaranteed to be executed without any further signals being caught.

If so instructed by the gl_get_line() configuration attributes of the signal that was caught, gl_get_line() restores the terminal attributes to the state that they had when gl_get_line() was called. This is particularly important for signals that suspend or terminate the process, since otherwise the terminal would be left in an unusable state.

It then reinstates the application's signal handlers.

Then it uses the C standard-library raise(3C) function to re-send the application the signal that was caught.

Next it unblocks delivery of the signal that we just sent. This results in the signal that was just sent by raise() being caught by the application's original signal handler, which can now handle it as it sees fit.

If the signal handler returns (that is, it does not terminate the process), gl_get_line() blocks delivery of the above signal again.

It then undoes any actions performed in the first of the above steps and redisplays the line, if the signal configuration calls for this.

gl_get_line() then either resumes trying to read a character, or aborts, depending on the configuration of the signal that was caught.

What the above steps do in essence is to take asynchronously delivered
signals and handle them synchronously, one at a time, at a point
in the code where gl_get_line() has complete control over its environment.

The Terminal Size

On most systems the combination of the TIOCGWINSZ ioctl and the SIGWINCH
signal is used to maintain an accurate idea of the terminal size.
The terminal size is newly queried every time that gl_get_line() is called
and whenever a SIGWINCH signal is received.

On the few systems where this mechanism is not available, at startup
new_GetLine() first looks for the LINES and COLUMNS environment variables. If these
are not found, or they contain unusable values, then if a terminal
information database like terminfo or termcap is available, the default size of the
terminal is looked up in this database. If this too fails to
provide the terminal size, a default size of 80 columns by 24
lines is used.

Even on systems that do support ioctl(TIOCGWINSZ), if the terminal is on
the other end of a serial line, the terminal driver generally has
no way of detecting when a resize occurs or of querying what
the current size is. In such cases no SIGWINCH is sent to the
process, and the dimensions returned by ioctl(TIOCGWINSZ) are not correct. The only
way to handle such instances is to provide a way for the
user to enter a command that tells the remote system what the
new size is. This command would then call the gl_set_term_size() function to
tell gl_get_line() about the change in size.

The ncolumn and nline arguments are used to specify the new dimensions
of the terminal, and must not be less than 1. On systems
that do support ioctl(TIOCGWINSZ), this function first calls ioctl(TIOCSWINSZ) to tell the terminal
driver about the change in size. In non-blocking server-I/O mode, if a
line is currently being input, the input line is then redrawn to
accomodate the changed size. Finally the new values are recorded in gl
for future use by gl_get_line().

The gl_terminal_size() function allows you to query the current size of the
terminal, and install an alternate fallback size for cases where the size
is not available. Beware that the terminal size will not be available
if reading from a pipe or a file, so the default values can
be important even on systems that do support ways of finding out
the terminal size.

This function first updates gl_get_line()'s fallback terminal dimensions, then records its findings
in the return value.

The def_ncolumn and def_nline arguments specify the default number of terminal columns
and lines to use if the terminal size cannot be determined by
ioctl(TIOCGWINSZ) or environment variables.

Hiding What You Type

When entering sensitive information, such as passwords, it is best not to
have the text that you are entering echoed on the terminal. Furthermore,
such text should not be recorded in the history list, since somebody
finding your terminal unattended could then recall it, or somebody snooping through your
directories could see it in your history file. With this in mind,
the gl_echo_mode() function allows you to toggle on and off the display
and archival of any text that is subsequently entered in calls to
gl_get_line().

The enable argument specifies whether entered text should be visible or not.
If it is 0, then subsequently entered lines will not be visible
on the terminal, and will not be recorded in the history list.
If it is 1, then subsequent input lines will be displayed as they
are entered, and provided that history has not been turned off with
a call to gl_toggle_history(), then they will also be archived in the
history list. Finally, if the enable argument is -1, then the echoing
mode is left unchanged, which allows you to non-destructively query the current setting
through the return value. In all cases, the return value of the
function is 0 if echoing was disabled before the function was called,
and 1 if it was enabled.

When echoing is turned off, note that although tab completion will invisibly
complete your prefix as far as possible, ambiguous completions will not be
displayed.

Single Character Queries

Using gl_get_line() to query the user for a single character reply, is
inconvenient for the user, since they must hit the enter or return
key before the character that they typed is returned to the program.
Thus the gl_query_char() function has been provided for single character queries like this.

This function displays the specified prompt at the start of a new
line, and waits for the user to type a character. When the
user types a character, gl_query_char() displays it to the right of the
prompt, starts a newline, then returns the character to the calling program. The
return value of the function is the character that was typed. If
the read had to be aborted for some reason, EOF is returned
instead. In the latter case, the application can call the previously documented
gl_return_status(), to find out what went wrong. This could, for example, have been
the reception of a signal, or the optional inactivity timer going off.

If the user simply hits enter, the value of the defchar argument
is substituted. This means that when the user hits either newline or
return, the character specified in defchar, is displayed after the prompt, as
though the user had typed it, as well as being returned to the
calling application. If such a replacement is not important, simply pass '\n'
as the value of defchar.

If the entered character is an unprintable character, it is displayed symbolically.
For example, control-A is displayed as ^A, and characters beyond 127 are
displayed in octal, preceded by a backslash.

As with gl_get_line(), echoing of the entered character can be disabled using
the gl_echo_mode() function.

If the calling process is suspended while waiting for the user to
type their response, the cursor is moved to the line following the
prompt line, then when the process resumes, the prompt is redisplayed, and
gl_query_char() resumes waiting for the user to type a character.

Note that in non-blocking server mode, if an incomplete input line is
in the process of being read when gl_query_char() is called, the partial
input line is discarded, and erased from the terminal, before the new
prompt is displayed. The next call to gl_get_line() will thus start editing a
new line.

Reading Raw Characters

Whereas the gl_query_char() function visibly prompts the user for a character, and
displays what they typed, the gl_read_char() function reads a signal character from
the user, without writing anything to the terminal, or perturbing any incompletely entered
input line. This means that it can be called not only from
between calls to gl_get_line(), but also from callback functions that the application
has registered to be called by gl_get_line().

On success, the return value of gl_read_char() is the character that was
read. On failure, EOF is returned, and the gl_return_status() function can be
called to find out what went wrong. Possibilities include the optional inactivity
timer going off, the receipt of a signal that is configured to abort
gl_get_line(), or terminal I/O blocking, when in non-blocking server-I/O mode.

Beware that certain keyboard keys, such as function keys, and cursor keys,
usually generate at least three characters each, so a single call to
gl_read_char() will not be enough to identify such keystrokes.

Clearing The Terminal

The calling program can clear the terminal by calling gl_erase_terminal(). In non-blocking
server-I/O mode, this function also arranges for the current input line to
be redrawn from scratch when gl_get_line() is next called.

Displaying Text Dynamically

Between calls to gl_get_line(), the gl_display_text() function provides a convenient way to
display paragraphs of text, left-justified and split over one or more terminal
lines according to the constraints of the current width of the terminal.
Examples of the use of this function may be found in the demo
programs, where it is used to display introductions. In those examples the
advanced use of optional prefixes, suffixes and filled lines to draw
a box around the text is also illustrated.

If gl is not currently connected to a terminal, for example if
the output of a program that uses gl_get_line() is being piped to
another program or redirected to a file, then the value of the
def_width parameter is used as the terminal width.

The indentation argument specifies the number of characters to use to indent
each line of ouput. The fill_char argument specifies the character that will
be used to perform this indentation.

The prefix argument can be either NULL or a string to place
at the beginning of each new line (after any indentation). Similarly, the
suffix argument can be either NULL or a string to place at the
end of each line. The suffix is placed flush against the right
edge of the terminal, and any space between its first character and
the last word on that line is filled with the character specified
by the fill_char argument. Normally the fill-character is a space.

The start argument tells gl_display_text() how many characters have already been written
to the current terminal line, and thus tells it the starting column
index of the cursor. Since the return value of gl_display_text() is the ending
column index of the cursor, by passing the return value of one
call to the start argument of the next call, a paragraph that
is broken between more than one string can be composed by calling
gl_display_text() for each successive portion of the paragraph. Note that literal newline characters
are necessary at the end of each paragraph to force a new
line to be started.

On error, gl_display_text() returns -1.

Callback Function Facilities

Unless otherwise stated, callback functions such as tab completion callbacks and event
callbacks should not call any functions in this module. The following functions,
however, are designed specifically to be used by callback functions.

Calling the gl_replace_prompt() function from a callback tells gl_get_line() to display a
different prompt when the callback returns. Except in non-blocking server mode, it
has no effect if used between calls to gl_get_line(). In non-blocking server mode,
when used between two calls to gl_get_line() that are operating on the
same input line, the current input line will be re-drawn with the
new prompt on the following call to gl_get_line().

International Character Sets

Since libtecla(3LIB) version 1.4.0, gl_get_line() has been 8-bit clean. This means that
all 8-bit characters that are printable in the user's current locale are now
displayed verbatim and included in the returned input line. Assuming that the
calling program correctly contains a call like the following,

setlocale(LC_CTYPE, "")

then the current locale is determined by the first of the environment
variables LC_CTYPE, LC_ALL, and LANG that is found to contain a valid
locale name. If none of these variables are defined, or the program
neglects to call setlocale(3C), then the default C locale is used, which
is US 7-bit ASCII. On most UNIX-like platforms, you can get a
list of valid locales by typing the command:

locale -a

at the shell prompt. Further documentation on how the user can make
use of this to enter international characters can be found in the
tecla(5) man page.

Thread Safety

Unfortunately neither terminfo nor termcap were designed to be reentrant, so you
cannot safely use the functions of the getline module in multiple threads
(you can use the separate file-expansion and word-completion modules in multiple
threads, see the corresponding man pages for details). However due to the use
of POSIX reentrant functions for looking up home directories, it is safe
to use this module from a single thread of a multi-threaded program,
provided that your other threads do not use any termcap or terminfo
functions.